Manufacturing plants that use chemicals often create, use or treat a process fluid. The process fluid typically is a gas or liquid flowing through various pieces of equipment including tanks, pipes, valves, and process equipment, at the manufacturing facility to make a product with the process fluid. Other manufacturing plants and facilities generate fluids as a by-product of the manufacturing process. Sometimes this by-product fluid has commercial value in and of itself, but often by-products are wasted to disposal sites. Prior to recovery and reuse of the process fluids, however, contaminants must be removed from the process fluid. Contaminants decrease the value of the process fluid, decrease its effectiveness, or otherwise interfere with its use either as a process fluid or for other uses as a by-product.
Landfills are one example of facilities that generate gas having commercial benefits. Landfills create methane gas as a by-product of disposal of waste. Previously, methane gas was allowed to vent freely to the atmosphere. Recently, however, efforts have been made to collect and use such gas. These efforts are made due in part to the economic value of the recovered methane gas and also to environmental purposes for improving air and water quality.
Various types of apparatus and equipment have been used for removing contaminants from process fluids. Scrubbers in various forms are often used to treat process fluids. Scrubbers include vertical scrubbers which pass the process fluid through filters which catch and hold the contaminants. Vertical scrubbers typically are counterflow in that the process fluid flows in one direction and the removed contaminants flow in an opposite or transverse direction. Other scrubbers are centrifugal and cyclonic separators. These devices induce spinning currents to the process fluid to separate contaminants from the useful process fluid.
Scrubbers and other devices for treating process fluids often use large quantities of absorbent fluids. The absorbent fluids are added to the process fluid for absorbing contaminants. Typically absorbent fluids are solvent which are relatively volatile. Federal government requirements, however, set standards for use and release of such materials, for the purpose of improving the quality of air and water. Special handling, controls, monitoring, and reporting on the solvents are often required.
Significant quantities of absorbent fluid are often typically used in treating a process fluid. Typically, the ratio of absorbent fluid to process fluid is 1:1 or higher. Often the weight of the solvent exceeds the weight of the process fluid to be treated. Such solvents incur other problems during disposal. Solvents are difficult to handle, use, store, and dispose with security in a waste facility.
One type of scrubber removes contaminants by means of physically contacting the process fluid with a solvent, typically in a compact heat transfer apparatus known as a heat exchanger. Heat exchangers include a plurality of heat transfer surfaces for exposing the heated process fluid to the absorption fluid which is usually chilled for improved solubility/absorption capacity. Compact heat exchangers are generally characterized as having extended surfaces for transfer of heat. The most common configurations are known in the industry as either a plate-fin or a tube-fin type of surface. The fins provide significantly large surface area-to-volume ratios to facilitate heat transfer. Although both types of heat exchangers function to extract heat from the process fluid, the types have significant differences. Of the two types, plate-fin devices typically exhibit a significantly higher ratio of heat transfer surface to volume of heat exchanger. The process fluid flowing through the heat exchanger typically is in relatively controlled flow against the heat transfer surfaces. The process fluid is divided into channels defined by the closely spaced separator plates such that specific flow regimes or flow patterns are created at the heat transfer surface with minimal eddy currents. The liquid film thickness on the plates and fins is minimized. Such flow enhances the transfer of heat from the process fluid to the refrigerant.
Tube-fin type heat exchangers in contrast typically exhibit relatively uncontrolled flow in the process fluid. This turbulence is in the form of uncontrolled cross-flow from one tube surface to an adjacent tube surface. The turbulent flow is imposed by the shape, spacing, and the relatively off-set relationship of the tubes that carry the refrigerant fluid. The resulting flow and non-uniform film boundary on the tubes and fins impedes transfer of heat from the process fluid to the refrigerant.
Typically, either type is used in a single stage heat exchanger designed for the specific cooling requirements. The heat exchanger provides the plurality of tubes or plates through which the refrigerant fluid flows. The refrigerant may be returned across the flow path of the process gas typically once or perhaps twice. The number of plates or tubes depend on the cooling capacity required.
Heat exchangers typically comprise a housing having a process fluid inlet and a process fluid outlet. The process fluid typically flows in a cross-direction to the refrigerant flow through the plates or tubes. A heat exchanger having two or more sections, or stages, of stacked plate or tube elements has been described previously. For example, U.S. Pat. No. 3,746,084 describes a housing that encloses several sections of stacked refrigerant pipes. Each section includes a separate input and output header for the refrigerant supplied to that section. This heat exchanger, however, does not provide a rigorous flow path for the process fluid. The header is exposed to the process fluid.
While functioning to extract heat from the process fluid, the heat exchanger has several drawbacks. Nonuniformity of heat extraction leads to purity problems of the resulting cooled process fluid. Heat exchangers particularly are used in condensers which extract waste liquid and contaminants and thereby purify the process fluid. The nonuniformity of the thermal transfer and the unrigorous flow path for the process fluid results in process fluid moving through the heat exchanger without being treated, e.g., some fluid does not contact enough transfer surface to effect cooling and absorption. Untreated process fluid retains the contaminants and thus the purity of the resulting process fluid is not as great. For example, methane gas typically emits from a land fill. The gas is generated by decaying debris placed in the landfill. The methane gas further contains contaminants that are emitted by the decaying materials. These contaminants include chlorinated hydrocarbons, aromatics, and organic silicon compounds, among others. The heat exchanger cools the methane gas to condense water from the gas. The contaminants condense and are absorbed into the water. The water drains from the heat exchanger to a collector. A separator may be used to divide the contaminants for separate collection.
Accordingly, there is a need in the art for an improved method of removing contaminants from a process gas.